1. Field of the Invention
The present invention relates to a damping coefficient control device for vibration dampers and, more particularly, to a damping coefficient control device for a vibration damper of the type disposed between a vibration generating section (vibration source) and a vibration receiving section and capable of damping vibration by viscous resistance of an electrorheologic fluid charged therein.
2. Description of the Related Art
Such a vibration damper has been known as having a liquid chamber a portion of which is constituted by an elastic material, as used in automobiles for mounting an engine, a carburetor and a body of a vehicle. This liquid chamber is sectioned into two small liquid sub-chambers by means of a partition wall. The liquid sub-chambers are communicated with each other by an orifice.
When the vibration damper is subjected to vibrations, the internal liquid contained in one of the liquid sub-chambers is moved to the other through the orifice, and the energy of the vibrations is absorbed by virtue of the resistance to passage through the orifice of the liquid.
In such a vibration damper, in order to cope with the vibrations of different frequencies which are generated in such a vibration source as an automotive engine, a plurality of orifices having different cross-sectional areas must be provided in such a manner that they are opened and closed independently of each other by such an opening and closing means as valves, resulting in a complicated construction.
In order to overcome this problem, a vibration damper has been proposed which employs an electrorheologic fluid as the vibration damping fluid. In operation, the viscosity of the fluid is changed by application of an electric field so as to enable the vibration damper to operate in response to different frequency levels of vibration. This type of vibration damper is disclosed in Japanese Patent Laid-Open Nos 104828/1985 and 74930/1986.
This type of vibration damper, however, is still unsatisfactory in the following respects. The first problem is that the vibration damper tends to resonate at the resonance frequency so that the vibration of the resonance frequency is undesirably transmitted at a higher rate than that of the other frequency ranges. Another problem is that the vibration damping performance deteriorates in the vibration damping range due to attenuation.